In a reciprocating internal combustion engine, in order to improve fuel economy by reducing the friction between the piston and the wall surface of the cylinder bore, it is desirable to increase the clearance between the piston and the wall surface of the cylinder bore (in other words, increase the piston clearance). However, when the clearance between the piston and the wall surface of the cylinder bore is increased, piston slap noise increases. Piston slap noise is caused by the collision of the skirt of the piston with the wall surface of the cylinder bore as a result of the swinging movement of the piston. Therefore, it is difficult to reduce friction and suppress piston slap noise at the same time.
In order to achieve these contradicting goals, Patent Document 1 proposes an improvement in the shape of the skirt of the piston. According to the piston disclosed in Patent Document 1, the skirt is provided with a barrel shape in such a manner that the radial projection of the skirt is maximized at a vertical position that is closer to the lower edge of the skirt than the center of the pin boss. However, the reduction of friction and the reduction of piston slap noise can be achieved only to a limited extent simply by adjusting the shape of the skirt of the piston.
Patent Document 2 proposes the use of a cylinder liner that defines a sliding surface of a cylinder bore which is configured such that the inner diameter of the part of the cylinder liner corresponding to the top ring at the top dead center of the piston is smaller than the inner diameter of the remaining part of the cylinder liner, and the inner diameter of the cylinder liner gets progressively greater as one moves from this point to the lower end thereof. According to this arrangement, oil is deposited in the top ring groove by using a suitable arrangement. As the piston moves toward the top dead center in the cylinder bore, the piston ring is pushed by the sliding surface of the cylinder bore into the ring groove. As a result, the oil deposited in the top ring can be pushed out into the gap between the sliding surface of the cylinder bore (sliding surface of the cylinder liner) and the piston to lubricate the sliding movement between the cylinder liner and the piston. Thus, the direct contact between the piston and the cylinder bore can be avoided so that the wear of the part of the sliding surface of the cylinder bore corresponding to the top ring at the top dead center of the piston can be avoided. However, piston slap noise caused by the collision between the skirt of the piston and the wall surface of the cylinder bore cannot be suppressed.